The invention relates to a method of editing an original sequence of data units (e.g., ADPCM-coded data units) that encodes an original audio signal e.g., a PCM-coded digital audio signal to generate an edited sequence representing a desired portion of the original audio signal. The edited sequence predominantly comprises predictive data units for reproduction by a predictive decoder whose response to each predictive unit is partially dependent upon an existing state of the decoder, which decoder is nevertheless responsive to a non-predictive type of data unit. The invention further relates to an apparatus for performing such a method.
Apparatuses for editing digitally coded audio signals are today well known in various applications, and can be constructed, for example, by using a high performance personal computer with appropriate software and interfaces. One known example comprises Digidesign's "Sound Tools" DSP hardware and software together with an Apple Macintosh (Registered Trade Mark) computer. The audio data in known apparatuses typically comprises PCM-coded sample values of 16-bits per sample for highest-quality sound. Such data is relatively easy to process, but takes a large account of storage per second of coded sound, demanding high performance hardware for an apparatus in terms of both storage capacity (hard disk space, typically) and channel bandwidth.
There is an increasing demand for sound to be reproduced by lower-cost apparatuses, and to this end compression techniques involving predictive coding have been developed. As one example, a variety of Adaptive Delta Pulse Code Modulation (ADPCM) coding has been developed for storage and retrieval of sound via Compact Disc-Read Only Memory (CD-ROM) in standardized systems known as CD-ROM-XA (CD-ROM Extended Architecture) and CD-I (Compact Disc-Interactive). A CD-ROM-XA player is described in U.S. Pat. No. 5,019,816. A CD-I player is described in "CD-I: A Designer's Overview", edited by Philips International and published by Kluwer Technical Books, ISBN 9020121103.
The predictive nature of ADPCM coding means that samples of an original 16-bit audio signal can be encoded with only a few bits per sample, because they are encoded by reference to preceding sample values held in a predictive decoder. A non-predictive type of data unit is typically selected only for high frequency signals, as part of an adaptive control strategy aimed at maximize instantaneous signal to noise ratio. ADPCM coding allow sound data (still of reasonably high quality) to be stored and processed by a low-cost apparatus such as a CD-I player. However, the predictive nature of the coding means that a portion of the audio signal cannot be decoded without reference back to a point when the state of the decoder was known, for example, the start of the recording. For this reason, editing of sounds for these lower cost systems is still performed with PCM data on high-performance systems, with conversion to ADPCM format occurring only when the finished sequence is ready for transfer to CD-ROM.
Apart from the general desire for lower cost editing systems, there is a growing desire for authoring of software for personal computers and systems such as CD-I to be performed on a target system itself so that end users can become authors, and authors can work on prototypes that resemble the finished products as closely as possible. From this viewpoint, an authoring system for CD-I applications ideally comprises a CD-I player, albeit expanded with necessary resources such as an interface to a magnetic hard disk drive for re-writable storage of authoring software tools, data files and programs under development. There remains, however, no path in such a system for real-time uncompressed PCM data to be edited and transferred in real time to the audio output for auditioning of source data and edited data. Continually decoding and recoding the audio data between PCM and ADPCM formats would be too slow for interactive use and would also degrade the quality of the signal.